This changes clutter_glx_texture_pixmap_update_area so it defers the
call to glXBindTexImageEXT until our pre "paint" signal handler which
makes clutter_glx_texture_pixmap_update_area cheap to call.
The hope is that mutter can switch to reporting raw damage updates to
ClutterGLXTexturePixmap and we can use these to queue clipped redraws.
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